Steps to Design a Vibrating Screen

These guidelines are nicely described in the Vibrating Screen Manufacturers Association handbook. Describes the exact reasons why for all of the important elements required to be taken into consideration when designing a vibrating screen.

The basic steps are many and here are but a few:

1. Determine what needs to be screened: material, wgt cu ft, tons per hour, max infeed size, product size wanted. 

2. This will give you base data to do a mathematic vibrating screen sizing (standard formula) to determine the size of the vibrating screen needed in square feet to do the job.

3. After determinig this factor, you must cross check the BED DEPTH to decide if it meets the guidelines of VSMA. This will either leave the size the same or require you to UPSIZE the machine footage to meet the bed depth needs. In other words, we DO NOT want to have 6" of material bed depth or mat at the end of the machine length for example on a 1/2" opening. This would result in carryover. RULE of thumb: WIDTH = More TPH and LENGTH = more clean or efficient screening

4. I have a chart of SPEED STROKE SLOPE ideal combinations for different screencloth openings for both INCLINED circle throw type machines that use 20 degree incline or gravity to help the foot travel of the material on the deck. This chart also covers FLAT or horizontal screens to identify the proper setup to screen at various openings. Rule of thumb: The stroke or eccentricity must be large enough always to CLEAR the TOP DECK openings first or nothing gets thru and nothing happens or in other words = ZERO SCREENING EFFICIENCY. The assumed efficiency of screening calculations is always 95% efficiency on top deck, and declines for the lower decks on the same machine.

5. Some typical SPEED STROKE SLOPE COMBOS for a 2 bearing free floating circle throw vibrating screen. We basically want to stay within 3.5 to 4.g's accelerating force to allow for efficient vibrating forces to actually screen good and not have the machine fly apart by operating at too high an accelerating force. This range of accelerating force basically refers to INCLINED CIRCLE THROW MACHINES working in the -3" +10mesh ranges.

OPG: +1" -3": need 3/8" circle, 850rpm, 18-25 deg range- 1"+4mesh: need 5/16" circle, 900rpm, 18-25 deg range-1/2"+10mesh need 1/4" circle, 1000rpm 18-25 deg range

Typical install angle is 20 degree and if material is non free flowing then, we make the angle either variable if on a portable screening plant or steeper if on a stationary plant. Typical FPM IN THE 60-75 FPM range.

6. The finer the screening the smaller the eccentric slope needed to stratify the coarse fractions and the faster the rpm is needed to make more introductions to the cloth openings. For very fine screening we go to 45 deg slope availability with a pin prick or very tiny 1/32" or smaller eccentric stroke with many different types of vibrating equipment specially

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designed to specifically handle these fine cuts. 

7. HORIZONTAL SCREENS: Run at higher G forces due to the fact we do not have MOTHER nature or gravity to help us with the FOOT TRAVEL rate OR FOOT PER MIN. (FPM) On a horizontal the typical fpm is 45 FPM.

8. Once we build the vibrating screen, with all the bells and whistles on it..........ie.....thicker liner plates, heavier than normal wirecloth, extended lips, heavy rubber vs wirecloth and take all the static weight into consideration - we now design the shaft and the eccentric offset to accomodate moving that static weight at the desired stroke or eccentric offset. The counterweights are typically a solid engineered mass that would have bolted on flywheel weights. These weights differ by manufacturer model.But, for example: It is not unusual for the flywheel to have three 1/2" plate weights bolted onto each wheel on the heavy side or six oclock position of the wheel. This would basically give us the largest stroke or circle you could get on the vibrating body. IE: 3/8" circle at 840 rpm. If i took 1 of plate weights off both wheels - it may reduce the stroke or circle size to 5/16" circle and then I would and could increase the speed of the motor sheave to run at 900 rpm, and if I took all the weights off the wheel completely - it may give me 1/4" circle which i could then speed up the machine to 1000 rpm by the rules of VSMA. RULE OF THUMB: always do 2 things..........not one. If you change stroke you should normally always change the speed. If i just sped up a unit that had a 3/8" circle size to 1000 RPM.......the accelerating forces would be way over the design factor and the sideplates and bearings would crack out or self destruct the machine.

Well I am not an actual designer of screen, however, following hints may be of some help.

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1) Decide amplitude and frequency, which can suit screening operation. This depends upon material and its size. 2) In general, more amplitude and lesser frequency for coarse screening. Contrary lesser amplitude and more frequency for finer screening. 3) Decide screen type based on 1 & 2 above. The type could be circular-throw, elliptical-throw, liner-throw, unbalanced mass, opposite revolving unbalanced masses, electromagnetic, etc. These can be intermixed. 4) Decide screen deck size in conjunction with desired screening efficiency.5) Decide material flow (movement) speed on deck, and corresponding material mass that would be resting on the deck. 6) Vibrating screen is a vibrating system. Thus, screen deck mass plus material mass on deck is total mass which is resting on springs. This is vibrated by exciting force. Mass, spring stiffness, damping factor (hysteresis losses) and exciting force are complementary; and together will create desired vibration. Proper design will result into desired screening, least efforts and least vibration to supporting-floor / house 

It is better to start with some reference existing model rather than from scratch. The knowledge of vibration engineering is must. The empirical reference data / information will be needed so that your design is in tune with present time, rather than at odd with well evolved present day design, since very long time development.

Above are only the main (brief) guidelines. 

Regards,Ishwar G Mulani.Author of Book : Engineering Science and Application Design for Belt Conveyor. Email : parimul@pn2.vsnl.net.in Tel.: 0091 (0)20 5882916

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